[PATCH v3 18/35] x86/bugs: Restructure srso mitigation

David Kaplan
2025-01-08T20:25:53Z

Restructure srso to use select/update/apply functions to create
consistent vulnerability handling. Like with retbleed, the command line
options directly select mitigations which can later be modified.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
---
arch/x86/kernel/cpu/bugs.c | 188 ++++++++++++++++++-------------------
1 file changed, 90 insertions(+), 98 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 08ac515df888..aee2945bdef9 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -84,6 +84,8 @@ static void __init srbds_select_mitigation(void);
static void __init srbds_apply_mitigation(void);
static void __init l1d_flush_select_mitigation(void);
static void __init srso_select_mitigation(void);
+static void __init srso_update_mitigation(void);
+static void __init srso_apply_mitigation(void);
static void __init gds_select_mitigation(void);
static void __init gds_apply_mitigation(void);
static void __init bhi_select_mitigation(void);
@@ -200,11 +202,6 @@ void __init cpu_select_mitigations(void)
rfds_select_mitigation();
srbds_select_mitigation();
l1d_flush_select_mitigation();
-
- /*
- * srso_select_mitigation() depends and must run after
- * retbleed_select_mitigation().
- */
srso_select_mitigation();
gds_select_mitigation();
bhi_select_mitigation();
@@ -220,6 +217,7 @@ void __init cpu_select_mitigations(void)
taa_update_mitigation();
mmio_update_mitigation();
rfds_update_mitigation();
+ srso_update_mitigation();

spectre_v1_apply_mitigation();
spectre_v2_apply_mitigation();
@@ -232,6 +230,7 @@ void __init cpu_select_mitigations(void)
mmio_apply_mitigation();
rfds_apply_mitigation();
srbds_apply_mitigation();
+ srso_apply_mitigation();
gds_apply_mitigation();
bhi_apply_mitigation();
}
@@ -2673,6 +2672,7 @@ early_param("l1tf", l1tf_cmdline);

enum srso_mitigation {
SRSO_MITIGATION_NONE,
+ SRSO_MITIGATION_AUTO,
SRSO_MITIGATION_UCODE_NEEDED,
SRSO_MITIGATION_SAFE_RET_UCODE_NEEDED,
SRSO_MITIGATION_MICROCODE,
@@ -2681,14 +2681,6 @@ enum srso_mitigation {
SRSO_MITIGATION_IBPB_ON_VMEXIT,
};

-enum srso_mitigation_cmd {
- SRSO_CMD_OFF,
- SRSO_CMD_MICROCODE,
- SRSO_CMD_SAFE_RET,
- SRSO_CMD_IBPB,
- SRSO_CMD_IBPB_ON_VMEXIT,
-};
-
static const char * const srso_strings[] = {
[SRSO_MITIGATION_NONE] = "Vulnerable",
[SRSO_MITIGATION_UCODE_NEEDED] = "Vulnerable: No microcode",
@@ -2699,8 +2691,7 @@ static const char * const srso_strings[] = {
[SRSO_MITIGATION_IBPB_ON_VMEXIT] = "Mitigation: IBPB on VMEXIT only"
};

-static enum srso_mitigation srso_mitigation __ro_after_init = SRSO_MITIGATION_NONE;
-static enum srso_mitigation_cmd srso_cmd __ro_after_init = SRSO_CMD_SAFE_RET;
+static enum srso_mitigation srso_mitigation __ro_after_init = SRSO_MITIGATION_AUTO;

static int __init srso_parse_cmdline(char *str)
{
@@ -2708,15 +2699,15 @@ static int __init srso_parse_cmdline(char *str)
return -EINVAL;

if (!strcmp(str, "off"))
- srso_cmd = SRSO_CMD_OFF;
+ srso_mitigation = SRSO_MITIGATION_NONE;
else if (!strcmp(str, "microcode"))
- srso_cmd = SRSO_CMD_MICROCODE;
+ srso_mitigation = SRSO_MITIGATION_MICROCODE;
else if (!strcmp(str, "safe-ret"))
- srso_cmd = SRSO_CMD_SAFE_RET;
+ srso_mitigation = SRSO_MITIGATION_SAFE_RET;
else if (!strcmp(str, "ibpb"))
- srso_cmd = SRSO_CMD_IBPB;
+ srso_mitigation = SRSO_MITIGATION_IBPB;
else if (!strcmp(str, "ibpb-vmexit"))
- srso_cmd = SRSO_CMD_IBPB_ON_VMEXIT;
+ srso_mitigation = SRSO_MITIGATION_IBPB_ON_VMEXIT;
else
pr_err("Ignoring unknown SRSO option (%s).", str);

@@ -2730,13 +2721,14 @@ static void __init srso_select_mitigation(void)
{
bool has_microcode = boot_cpu_has(X86_FEATURE_IBPB_BRTYPE);

- if (!boot_cpu_has_bug(X86_BUG_SRSO) ||
- cpu_mitigations_off() ||
- srso_cmd == SRSO_CMD_OFF) {
- if (boot_cpu_has(X86_FEATURE_SBPB))
- x86_pred_cmd = PRED_CMD_SBPB;
+ if (!boot_cpu_has_bug(X86_BUG_SRSO) || cpu_mitigations_off())
+ srso_mitigation = SRSO_MITIGATION_NONE;
+
+ if (srso_mitigation == SRSO_MITIGATION_NONE)
return;
- }
+
+ if (srso_mitigation == SRSO_MITIGATION_AUTO)
+ srso_mitigation = SRSO_MITIGATION_SAFE_RET;

if (has_microcode) {
/*
@@ -2749,98 +2741,98 @@ static void __init srso_select_mitigation(void)
setup_force_cpu_cap(X86_FEATURE_SRSO_NO);
return;
}
-
- if (retbleed_mitigation == RETBLEED_MITIGATION_IBPB) {
- srso_mitigation = SRSO_MITIGATION_IBPB;
- goto out;
- }
} else {
pr_warn("IBPB-extending microcode not applied!\n");
pr_warn(SRSO_NOTICE);

- /* may be overwritten by SRSO_CMD_SAFE_RET below */
- srso_mitigation = SRSO_MITIGATION_UCODE_NEEDED;
+ /* Fall-back to Safe-RET */
+ srso_mitigation = SRSO_MITIGATION_SAFE_RET_UCODE_NEEDED;
}

- switch (srso_cmd) {
- case SRSO_CMD_MICROCODE:
- if (has_microcode) {
- srso_mitigation = SRSO_MITIGATION_MICROCODE;
- pr_warn(SRSO_NOTICE);
- }
+ switch (srso_mitigation) {
+ case SRSO_MITIGATION_MICROCODE:
break;

- case SRSO_CMD_SAFE_RET:
- if (boot_cpu_has(X86_FEATURE_SRSO_USER_KERNEL_NO))
- goto ibpb_on_vmexit;
-
- if (IS_ENABLED(CONFIG_MITIGATION_SRSO)) {
- /*
- * Enable the return thunk for generated code
- * like ftrace, static_call, etc.
- */
- setup_force_cpu_cap(X86_FEATURE_RETHUNK);
- setup_force_cpu_cap(X86_FEATURE_UNRET);
-
- if (boot_cpu_data.x86 == 0x19) {
- setup_force_cpu_cap(X86_FEATURE_SRSO_ALIAS);
- x86_return_thunk = srso_alias_return_thunk;
- } else {
- setup_force_cpu_cap(X86_FEATURE_SRSO);
- x86_return_thunk = srso_return_thunk;
- }
- if (has_microcode)
- srso_mitigation = SRSO_MITIGATION_SAFE_RET;
- else
- srso_mitigation = SRSO_MITIGATION_SAFE_RET_UCODE_NEEDED;
- } else {
+ case SRSO_MITIGATION_SAFE_RET:
+ case SRSO_MITIGATION_SAFE_RET_UCODE_NEEDED:
+ if (!IS_ENABLED(CONFIG_MITIGATION_SRSO))
pr_err("WARNING: kernel not compiled with MITIGATION_SRSO.\n");
- }
+ else if (boot_cpu_has(X86_FEATURE_SRSO_USER_KERNEL_NO))
+ srso_mitigation = SRSO_MITIGATION_IBPB_ON_VMEXIT;
break;

- case SRSO_CMD_IBPB:
- if (IS_ENABLED(CONFIG_MITIGATION_IBPB_ENTRY)) {
- if (has_microcode) {
- setup_force_cpu_cap(X86_FEATURE_ENTRY_IBPB);
- srso_mitigation = SRSO_MITIGATION_IBPB;
-
- /*
- * IBPB on entry already obviates the need for
- * software-based untraining so clear those in case some
- * other mitigation like Retbleed has selected them.
- */
- setup_clear_cpu_cap(X86_FEATURE_UNRET);
- setup_clear_cpu_cap(X86_FEATURE_RETHUNK);
- }
- } else {
+ case SRSO_MITIGATION_IBPB:
+ if (!IS_ENABLED(CONFIG_MITIGATION_IBPB_ENTRY))
pr_err("WARNING: kernel not compiled with MITIGATION_IBPB_ENTRY.\n");
- }
break;

-ibpb_on_vmexit:
- case SRSO_CMD_IBPB_ON_VMEXIT:
- if (IS_ENABLED(CONFIG_MITIGATION_SRSO)) {
- if (!boot_cpu_has(X86_FEATURE_ENTRY_IBPB) && has_microcode) {
- setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
- srso_mitigation = SRSO_MITIGATION_IBPB_ON_VMEXIT;
-
- /*
- * There is no need for RSB filling: entry_ibpb() ensures
- * all predictions, including the RSB, are invalidated,
- * regardless of IBPB implementation.
- */
- setup_clear_cpu_cap(X86_FEATURE_RSB_VMEXIT);
- }
- } else {
+ case SRSO_MITIGATION_IBPB_ON_VMEXIT:
+ if (!IS_ENABLED(CONFIG_MITIGATION_SRSO))
pr_err("WARNING: kernel not compiled with MITIGATION_SRSO.\n");
- }
+ break;
+ default:
+ break;
+ }
+}
+
+static void __init srso_update_mitigation(void)
+{
+ /* If retbleed is using IBPB, that works for SRSO as well */
+ if (retbleed_mitigation == RETBLEED_MITIGATION_IBPB)
+ srso_mitigation = SRSO_MITIGATION_IBPB;
+
+ if (srso_mitigation != SRSO_MITIGATION_NONE)
+ pr_info("%s\n", srso_strings[srso_mitigation]);
+}
+
+static void __init srso_apply_mitigation(void)
+{
+ if (srso_mitigation == SRSO_MITIGATION_NONE) {
+ if (boot_cpu_has(X86_FEATURE_SBPB))
+ x86_pred_cmd = PRED_CMD_SBPB;
+ return;
+ }
+ switch (srso_mitigation) {
+ case SRSO_MITIGATION_SAFE_RET:
+ case SRSO_MITIGATION_SAFE_RET_UCODE_NEEDED:
+ /*
+ * Enable the return thunk for generated code
+ * like ftrace, static_call, etc.
+ */
+ setup_force_cpu_cap(X86_FEATURE_RETHUNK);
+ setup_force_cpu_cap(X86_FEATURE_UNRET);
+
+ if (boot_cpu_data.x86 == 0x19) {
+ setup_force_cpu_cap(X86_FEATURE_SRSO_ALIAS);
+ x86_return_thunk = srso_alias_return_thunk;
+ } else {
+ setup_force_cpu_cap(X86_FEATURE_SRSO);
+ x86_return_thunk = srso_return_thunk;
+ }
+ break;
+ case SRSO_MITIGATION_IBPB:
+ setup_force_cpu_cap(X86_FEATURE_ENTRY_IBPB);
+ /*
+ * IBPB on entry already obviates the need for
+ * software-based untraining so clear those in case some
+ * other mitigation like Retbleed has selected them.
+ */
+ setup_clear_cpu_cap(X86_FEATURE_UNRET);
+ setup_clear_cpu_cap(X86_FEATURE_RETHUNK);
+ break;
+ case SRSO_MITIGATION_IBPB_ON_VMEXIT:
+ setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
+ /*
+ * There is no need for RSB filling: entry_ibpb() ensures
+ * all predictions, including the RSB, are invalidated,
+ * regardless of IBPB implementation.
+ */
+ setup_clear_cpu_cap(X86_FEATURE_RSB_VMEXIT);
break;
default:
break;
}

-out:
- pr_info("%s\n", srso_strings[srso_mitigation]);
}

#undef pr_fmt
--
2.34.1